Most permafrost in
the Northern Hemisphere occurs between latitudes of 60°N and 68°N. (North of
67°N, permafrost declines sharply, as the exposed land surface gives way to the
Arctic Ocean.) There is also a significant amount of permafrost around 35°N, in
the Qinghai-Xizang (Tibet) Plateau, and in the mountains of southwest Asia and
the U.S. Rocky Mountains.

Permafrost is not
defined by soil moisture content, overlying snow cover, or location; it's
defined solely by temperature. Any rock or soil remaining at or below 0°C (32°
F) or two or more years is permafrost. Permafrost can contain over 30 percent
ice, or practically no ice at all. It can be overlain by several meters of snow,
or little or no snow. Understanding permafrost is not only important to civil
engineering and architecture, it's also a crucial part of studying global change
and protecting the environment in cold regions.

What are the types of frozen
ground? Frozen ground is can be either seasonally frozen ground or permafrost.
Seasonally frozen ground freezes in the winter and thaws in the summer. More
than half of the land in the Northern Hemisphere has some seasonally frozen
ground.

Permafrost is a type of
frozen ground that stays at or below 0° Celsius (32° Fahrenheit) for at
least two years. Permafrost does not have to contain water or ice. As long
as the temperature of the ground stays below freezing, it is still
considered frozen ground, even if it is completely dry. If permafrost
begins to warm significantly, it thaws.

These two types of frozen
ground can occur separately, or together. A layer of ground that freezes
and thaws every year may sit on top of permafrost. This is called the
active layer. The active layer is seasonally frozen ground, and is not
part of the permafrost. Permafrost begins where the seasonally frozen
ground ends.

A cross-section of permafrost shows an ice wedge hiding just below the surface

Credit: U.S. Fish and Wildlife Service

Permafrost Types

Continuous permafrost
Continuous permafrost exists under almost the entire land surface in an
area. Areas with continuous permafrost often have permafrost layers more
than 100 meters (330 feet) thick. The deepest permafrost ever found is in
Siberia, a region in northern Russia. One area in Siberia has a permafrost
layer that extends down 1,650 meters (5,413 feet).

Discontinuous permafrost
Discontinuous permafrost exists under a large portion of a particular area
or only in a few specific places. Alpine permafrost is discontinuous
permafrost that exists on the tops of mountains where the ground stays
very cold. In areas with discontinuous permafrost, the permafrost layer
may extend as deep as ten meters (thirty-three feet) underground. Taliks
are sections of unfrozen ground within permafrost.

Discontinuous permafrost
can be isolated or sporadic. It is called isolated if less than ten
percent of the surface has permafrost under it. Sporadic means ten to
fifty percent of the surface has permafrost under it.

Cold
permafrost — Remains below 30° F, and which may be as low as 10° F as on the North
Slope; tolerates introduction of considerable heat without thawing.

Ice-rich
— 20% to 50% visible ice.

Thaw-stable
— Permafrost in bedrock, in well drained, coarse-grained sediments such as
glacial outwash gravel, and in many sand and gravel mixtures. Subsidence or
settlement when thawed is minor, foundation remains essentially sound.

Thaw-unstable
— Poorly drained, fine grained soils, especially silts and clays. Such soils
generally contain large amounts of ice. The result of thawing can be loss of
strength, excessive settlement and soil containing so much moisture that it
flows.

This map shows the
location of different permafrost types in Alaska. Darker shades of blue indicate
larger percentages of permanently frozen ground. Lighter blues, and the terms
isolated and sporadic, refer to lower percentages of frozen ground. White
indicates areas where permafrost formed during the last ice age, in areas where
conditions would not allow it to form today. This permafrost is called relict
permafrost.Credit:NSIDC

Most frozen ground
in the United States is seasonally frozen ground. The shaded areas on the map,
between the dotted lines, mark the average depth that the soil freezes to in the
winter. The actual depth that the ground freezes to in any particular place may
vary due to local climate, soil type, elevation, and terrain. —Credit: Neil
Davis NSIDC

Proximity to large
water bodies tends to reduce temperature extremes, which affects the
distribution of permafrost. Scandinavia and Iceland, for instance, have
relatively little permafrost . About 37 percent of Northern Hemisphere
permafrost occurs in western North America, mainly in Alaska and northern Canada
between 165°W and 60°W. Most permafrost occurs in the Eastern Hemisphere, mainly
in Siberia and the Far East of Russia, northern Mongolia, northeastern China,
the Qinghai-Xizang (Tibet) Plateau, and surrounding mountains between 60°E and
180°E

Distribution of permafrost and ground ice in the Northern Hemisphere, based on
the EASE-Grid version of the IPA map. "High," "Med" and "Low" refer to ice
content, and "T" and "t" refer to thick and thin overburden, respectively.
National Snow and Ice Data Center Graphic

Despite its name,
permafrost is characterized by its instability. It is often covered by an active
layer that regularly melts. Although permafrost can be thousands of years old,
it is sometimes newly formed or about to melt, and it often exists close to its
melting point . As permafrost thaws, it jeopardizes both man-made structures and
natural features. Thawing permafrost on mountain slopes can lead to landslides .
Approximately 55 percent of the Northern Hemisphere's land surface is covered by
seasonally frozen ground, which can last for a few weeks in the middle and lower
latitudes, and for several months at high latitudes and high elevations .

An illustration of the range in temperatures experienced at different depths in
the ground during the year. The active layer (shown in grey) thaws each summer
and freezes each winter, while the permafrost layer remains below 0°C.

Much of the Northern Hemisphere frozen ground is overlain by evergreen boreal
forest. These boreal forests comprise both a source and a sink of carbon. In
fact, the Arctic contains nearly one-third of the Earth's stored soil carbon. If
the high northern latitudes were to have a significant temperature increase, the
regional soils would begin to release carbon into the atmosphere, which could
lead to increased plant growth, carbon aspiration, and possibly a temperature
drop or stabilization. Alternately, it could lead to higher temperatures,
fueling the cycle of carbon release and temperature rise .

Frozen ground's
widespread distribution makes it a substantial component of the cryosphere.
Likewise, its role in the storage and release of carbon make it a major factor
in future global change.

General
circulation models predict that, for a doubling of atmospheric
concentrations of carbon dioxide due to anthropogenic sources, mean annual
air temperatures may rise up to several degrees over much of the Arctic.
In the discontinuous permafrost region, where ground temperatures are
within 1-2 degrees of melting, permafrost will likely ultimately disappear
as a result of ground thermal changes associated with global climate
warming. Where ground ice contents are high, this permafrost degradation
will have associated physical impacts. Of greatest concern are soils with
the potential for instability upon thaw (thaw settlement, creep or slope
failure). Such instabilities may have implications for the landscape,
ecosystems, and infrastructure.

Siberia feels the heat of global warming Russia Today Video

Credit NASA, UNEP, The National Snow and Ice Data Center, University of Alaska
at Fairbanks, International Permafrost Association, Natural Resources Canada
Geological Survey

Data compiled from The
British Antarctic Study, NASA, Environment Canada, UNEP, EPA and
other sources as stated and credited Researched by Charles
Welch-Updated daily This Website is a project of the The Ozooe Hole
Inc. http://www.theozonehole.com